1. Field of the Invention
The present invention relates generally to air filters of foam type construction and, more specifically, to an improved foam filter and method of construction which provides a foam filter having increased efficiency while maintaining a desired filter capacity and without adversely affecting the restriction characteristics of the filter.
2. Description of the Prior Art
In a polluted world, efficient air filters perform an important function. These filters clean the air of particulate matter that can clog and corrode expensive equipment, such as automobile engines, furnaces and air conditioners. In automobiles, air filters must filter out the maximum quantity of air pollutants without decreasing or restricting the amount of air available to the vehicle for combustion. In a furnace or air conditioner, the air filter traps particulates from the air prior to heating or cooling the air and blowing the heated or cooled air throughout the house.
Currently, automotive air filters are typically made of pleated paper housed in a plastic or metal frame. They are used once and then discarded. The paper, however, is limited in its ability to remove particulate matter. In the age of self-service gas stations and vehicles without carburetors that require infrequent tune-ups, car owners frequently neglect to check under the hood and remove dirty filters. These dirty filters hinder engine performance and contribute to engine destruction.
Foam air filters can be used to overcome certain of the deficiencies associated with the prior art pleated paper filters. In particular, "reticulated" foams can be produced which offer the possibility of filtering efficiencies not previously achievable with the pleated paper type air filters. Reticulated foams of the type under consideration are generally described in the literature as being of either the "polyether" or "polyester" type. Each type of prior art foam has certain desirable and undesirable characteristics. For example, polyester foams can currently be manufactured having the smaller porosities needed for certain applications, such as for use in automotive filters. Polyester foams are currently available in the pore size range from about 3 pores per inch (ppi) to about 90-110 ppi. Heat and moisture, unfortunately, hydrolytically degrade the prior art polyester foams, making them unsuitable for some applications. Polyether foams are less affected by elevated temperature and moisture conditions. Thus, while the polyether foams are less susceptible to hydrolytic degradation than are polyester foams, the polyether foams have generally been commercially available only in larger pore size ranges. Typical commercially available polyether foams are available in pore sizes ranging from about 3 ppi to about 70-80 ppi. These pore sizes are not small enough for some applications.
Dry foams filter like a sieve under dry conditions. The prior art has recognized that, if a low viscosity oil treatment is applied to a dry foam filter, the filter action changes and the efficiency of filtering increases. The air filters on many lawn and garden engines have consequently been treated with lower viscosity oils such as motor oils, in the past. The prior art oil treatments have proved problematical in many cases, however, in that the oil tended to migrate by gravity flow within the filter medium. This left one region of the filter relatively dry and subsequently less efficient in operation while another region of the filter was saturated or over-treated. Additionally, many of the prior art lower viscosity oils are lost from the foam filter during use, as where the filter medium is exposed to increasing air flow velocity.
One object of the present invention is to produce a foam type air filter that resists hydrolytic degradation while it efficiently removes airborne particulate matter without substantially restricting air passage.
Another object of the invention is to treat a foam type air filter with a controlled migration treatment medium which increases the filter efficiency and yet which does not migrate through the filter in an unacceptable fashion.
Another object of the invention is to treat a polyester type foam filter to provide a filter which is highly resistant to water and which is stable under elevated temperature conditions, thereby allowing the use of a smaller cell sized foam for needed applications.
Another object of the invention is to provide a foam filter having a controlled migration treatment medium and multiple layer construction in order to provide a working filter having the highest capacity, highest efficiency and lowest restriction possible.